There are a variety of applications for audio systems using a substantial number of microphones or similar audio signal sources. In the entertainment field, a plurality of microphones may be employed to cover a large stage area and, in some instances, portions of the audience. Another application for a multiple-microphone audio system is in the board room for a board having a large number of members. A legislative body affords another application.
In any such audio system, available acoustic gain must be limited to preclude the howling effect that can be produced by feedback. Using an ordinary mixer arrangement, with no priority control, a system incorporating ten microphones must be limited to 10 dB less gain than a single microphone system operating under the same conditions in order to prevent excessive feedback. That is, the addition of microphones to a system generally requires that the gain be reduced in accordance with the increased tendency toward feedback effects. A second drawback of an audio system utilizing a substantial number of microphones is the increased tendency toward pickup of undesirable background noise with subsequent amplification by the system.
Traditionally, systems incorporating substantial numbers of microphones have been operated at a marginally useful gain level. Alternatively, a technician has been employed to vary the gain of individual audio channels or of the entire system, judiciously fading microphones up and down as necessary.
Several different mixer controls have been proposed and utilized in an effort to improve upon the traditional arrangements for multiple microphone audio systems. Thus, a voice-operated microphone control can be used to limit the number of microphones that are effectively "on". In a system of this kind, for a given microphone to get "on" the voice level at the microphone must exceed a preset threshold. All microphones above the threshold are on and all below are held off. But the threshold setting is highly critical. If the threshold is set too low, background noise may turn one or more microphones on, producing undesirable amplified noise; if the threshold is set too high, persons who speak softly may be denied access to the system or may have their speech chopped. Moreover, if the system gain is set to accommodate one or two microphones on before feedback occurs, a loud sound may turn a number of microphones or even all microphones on and may latch them into a continuing feedback condition.
To avoid the latched feedback mode in voice-operated microphone systems, controls have been devised which operate to lock out all remaining microphones once a single microphone gains access to the system. In these controls, the microphones are usually scanned sequentially to find one that is above a given threshold. That microphone is then effectively turned "on", and is held "on" until its signal level drops below the threshold, at which time the control resumes its scan to find another microphone operating above threshold. This control provides maximum possible gain before feedback without fear of several microphones coming on. Its principal disadvantage is that conversations between two or more speakers are frequently chopped, particularly at the beginning of words or at the end of pauses. In conversational exchanges, when people frequently speak simultaneously or respond rapidly, noticeable word loss often occurs. Furthermore, the use of a high threshold may cause soft voices to be missed. Again, however, if the threshold is lowered, extraneous noise can interrupt the scan and prevent a bona fide active microphone from gaining access to the system.
Another modification of voice-operated controls, which allows two or more microphones to have access to the system simultaneously without increasing the danger of excessive feedback, is the NOM (number of open microphones) master gain attenuator. In a control of this kind, as additional microphones cross the threshold, the number of microphones currently "on" is counted and used to reduce the total system gain and avoid excessive feedback. Thus, if two microphones are "on" the gain is reduced by 3 dB; if ten microphones are "on," the gain reduction is 10 dB. This method allows multiple voice conversations with only transitory gain reductions, which are usually unnoticeable. However, the same threshold problems still persist. A low threshold allows many microphones "on," often due to background noise, with accompanying reduction of system gain, whereas a high threshold prevents weak voices from establishing access to the system.
In somewhat different controls, as presented in Dugan U.S. Pat. Nos. 3,814,856 and 3,992,584, the use of a preset threshold is eliminated. The relative output levels of the microphones are compared and system gain is apportioned to the microphones in accordance with their individual output levels. Thus, the microphone having the highest initial output level receives the most gain and the microphone having the lowest output receives the least gain. The overall gain of the system is held substantially constant to minimize feedback problems. Certain difficulties and disadvantages remain, however. If a soft voice and a loud voice are competing for use of the system, the louder voice tends to overshadow the softer voice disproportionately. Background noise is picked up and amplified in much the same manner as a standard mixer amplifier. Moreover, these controls require continuous comparison of the signals from the microphones over a very wide dynamic range, taxing the accuracy and stability of available circuits.
Another approach is presented in Nicholas et al U.S. Pat. No. 3,947,639 and Nicholas U.S. Pat. No. 3,958,084, directed to telephone conference systems. In those systems, one active source is selected for each conferee, based on amplitude, from all other conferees; the peak signal level for that active source establishes a variable reference which another source must exceed to become the new active source. The reference decays and is renewed on a fixed cyclic basis. These controls, like many voice-operated systems, are limited to one source "on," for each conferee, at any given time. Chopping remains a distinct possibility, and there is no way for any source to gain access to the system when a louder source is present.